update m5stack gray en zh ja

docs/en/core/gray.md

@@ -106,7 +106,7 @@ This is a [Bottom board](en/base/core_bottom) on the back of M5Core. It's design
    </tr>
    <tr>
       <td>Flash</td>
-      <td>4MB</td>
+      <td>16MB (old model: 4MB)</td>
    </tr>
    <tr>
       <td>Input</td>

docs/ja/core/gray.md

@@ -109,7 +109,7 @@
    </tr>
    <tr>
       <td>Flash</td>
-      <td>4MB</td>
+      <td>16MB (旧: 4MB)</td>
    </tr>
    <tr>
       <td>電源入力</td>

docs/zh_CN/core/gray.md

@@ -82,7 +82,7 @@
    </tr>
    <tr>
       <td>Flash闪存</td>
-      <td>4M-Bytes</td>
+      <td>16M-Bytes (旧: 4M-Bytes)</td>
    </tr>
    <tr>
       <td>输入电压</td>